Different techniques are known to manufacture resistors by bringing a non-insulating, electrically resistive film or foil material, such as metal film or metal foil, for example, nickel chromium, cermet film, for example, tantalum nitride, ruthenium dioxide, bismuth ruthenate, carbon film, or a film of composite material based on a mixture of glass and cermet onto an insulating substrate. In rare cases, the electrically resistive film material may consist of multiple, different layers of the above-mentioned materials. The insulating substrate may be ceramic, silicon, glass or some other synthetic material, and the film material is applied to the substrate by methods such as sputtering (thin film), screen and stencil printing (thick film) or direct printing through a nozzle (thick film). The insulating substrate may have the form of a flat planar sheet or of a cylinder, and accordingly the resistive film is deposited either onto a two-dimensional planar surface or onto a three-dimensional axially symmetric surface. In the voltage divider, both the high and low ohmic resistors are brought onto the same substrate. In addition, highly conductive structures with considerable lower resistivity than the film material of the resistors are deposited on the substrate as well. The highly conductive structures are intended to be used as contacting terminals, and they are placed on the substrate in such a way that the resistive film material of the resistors overlaps partly with them.
In order to achieve voltage ratios of significantly more than unity and at the same time reduce the size of the voltage divider, it is known to arrange the resistive film material of the high ohmic resistor in a long and narrow trace, where the trace is shaped like a meandering form. The term meandering form means that the trace is not just a straight line but curved in such a way that a long length is achieved on a small substrate area. The meandering form may look for example like a square wave, a triangle wave, a sine wave, a zigzag or—in the three-dimensional case—a helical form. This is, for example, described in U.S. Pat. No. 5,521,576 for thick film resistors and in U.S. Pat. No. 7,079,004 B2 for thin film AC voltage dividers. As is disclosed there as well, the low resistance value of the low ohmic resistor is commonly obtained by arranging the resistive film material in a short and wide trace.
In general, the above-described resistive voltage dividers can be used for a wide range of voltage levels, from low or medium up to high voltage applications. While the present disclosure originates from the area of medium voltage sensors, such as the KEVCD and KEVA sensor types by ABB, which are commonly applicable to a voltage range between 3.6 kV and 36 kV, the area of application of the present disclosure is not limited to this voltage range.
For most applications, it is desirable that a certain high accuracy of the ratio of the voltage divider can be ensured and that the initial accuracy can be maintained over changing temperature and/or over a long period of time. However, different ageing effects lead to a drift in both the resistance value and the temperature coefficient of the resistors in the divider which results in a gradual deterioration of the accuracy. In U.S. Pat. No. 5,521,576, it is described that for thick film resistors, stable operating characteristics of the resistors can be ensured at a desired value of resistance by printing the resistive composition of the resistive line directly onto the substrate in a continuous pattern, where the resistive line has a length which is at least ten times larger than the width of the line.
The inventors of the present disclosure have realized that for voltage dividers, it is not so much the resistance value of each resistor itself which needs to be stabilized. Instead, it is more important to ensure that the initial resistance values of the resistors are accurate and that the drifting of the operating characteristics of both the high and the low ohmic resistors occurs in the same direction and with possibly the same amount so that the ratio of the resistance values and thereby the divider's voltage ratio is maintained at its initial value over the whole operating temperature range as well as over a long period of time.